Reaction propulsion engine and method of operation

ABSTRACT

1. A reaction propulsion system including means providing a combustion chamber having at its rearward end an impulse expansion outlet nozzle, means providing a ram air intake, means directing air from the ram air intake to the forward end of the combustion chamber, a fuel storage chamber, indirect heat exchange means in heat exchange contact with the ram air in said air directing means, means directing fuel from said storage chamber through said heat exchange means, an air compressor providing a portion of the air directing means between the heat exchange means and the combustion chamber, a direct expansion turbine for driving said compressor, a regenerative heat exchanger positioned in an auxiliary combustion chamber, means for selectively directing at least a portion of the fuel passing through said heat exchange means to said regenerative heat exchanger, means directing at least a portion of the fuel from said heat exchange means and the fuel passing through the regenerative heat exchanger through the direct expansion turbine, and means for selectively combusting the fuel exhausting from the direct expansion turbine in the combustion chamber and in the auxiliary combustion chamber housing the regenerative heat exchanger.

a United States Patent Wolf et al.

[ Dec. 12, 1972 i5 1 REACTION PROPULSION ENGINE AND Rae .11: ..60/246 Primary Examiner-Samuel Feinberg Attorney-Stowell and Stowell EXEMPLARY CLAIM l. A reaction propulsion system including means providing a combustion chamber having at its rearward end an impulse expansion outlet nozzle, means providing a ram air intake, means directing air from the ram air intake to the forward end of the combustion chamber, a fuel storage chamber, indirect heat exchange means in heat exchange contact with the ram air in said air directing means, means directing fuel from said storage chamber through said heat exchange means, an air compressor providing a portion of the air directing means between the heat exchange means and the combustion chamber, a direct'expansion turbine for driving said compressor, a regenerative heat exchanger positioned in an auxiliary combustion chamber, means for selectively directing at least a portion of the fuel passing through said heat exchange means to said regenerative heat exchanger, means directing at least a portion of the fuel from said heat exchange means and the fuel passing through the regenerative heat exchanger through the direct expansion turbine, and means for selectively combusting the fuel exhausting from the direct expansion turbine in the combustion chamber and in the auxiliary combustion chamber housing the regenerative heat exchanger.

4 Claims, 4 Drawing Figures 3,705,496 SHEEI 1 OF 3 PATENTED 05c 12 m2 INVENTOR ROBERT L. WOLF RODNEY MC GANN S. fi

ATTURNI'IY PATENIEU um 12 m2 SHEET 2 BF 3 INVENTOR ROBERT L. WOLF RODNEY Mc GANN ATTORNEYS PATENTEDuzc 12 m2 SHEET 3 OF 3 R .O T m x25 o T N 4 1/ xiii!!! TuHH m u 5 l/4 H wm lillillllli ROBERT L. WOLF RODNEY Mc GANN ATTORNEYS This invention relates to reaction propulsion engines and to methods of operating them and particularly to relatively small and lightweight air breathing reaction propulsion engines which will be able to accelerate efficiently a load from standstill to hypersonic speeds.

It is principal object of the present invention to provide an air breathing reaction engine wherein a portion of the energy of the ram air is transferred to the fuel by heat exchange between the ram air and the fuel and wherein further energy may be transferred to the fuel by heat exchange between the fuel and the products of combustion of the engine and thereafter the energy thus transferred is at least in partutilized by expanding the helm a portion thereof in a turbine driving an air compressor for the engine.-

By locating a heat exchanger in the air inlet, the heat exchanger cools theingested diffused air so that the engine downstream of the heat exchanger will not be subjected to temperature exceeding, forexample, that corresponding to about Mach 3 (about 650F) during Mach 8 flight conditions. I I v v v A further object is to provide such an engine wherein the air stream is more easily compressed by cooling the ingested air prior to compression.

' Another object of the present invention is to provide an engine having. a regenerative heat exchanger in a combustion zone of the engine whereby from launching velocities to the low hypersonic range of about Mach 4 the regenerative heat exchanger is employed to provide a portion of the energy required to operate the air compressor.

Another object of the invention is to provide an engine wherein the turbine is not aerodynamically coupled to the compressor whereby the power supply to the turbine does not depend on air flow through the compressor eliminating the need for variable geometry compressors or air bleeding while operating over a very wide range of flight conditions.

Anotherobject of the present invention is to provide an air turbo engine wherein the air bypasses the turbine and the full compressor discharge pressure is discharged into the combustion zone of the engine.

For high cooling efficiency, it is desirable that the fuel selected have a high heat capacity within the operating temperature range to provide for a high absorption of energy per pound of fuel. For efficient conversion of energy to thrust in the expansion process, the fuel should be one which provides a low average molecular weight in the exhaust gases. Within these limitations a considerable range of fuels is available from which selection may be made in the light of other properties of the fuels and the effect on the particular demands to be made on the engine. Among the fuels of interest are hydrogen and hydrogen-bearing compounds such as ammonia, methyl alcohol, ethyl alcohol, methane, ethylene glycol and cyclohexane.

Of these fuels, fuels which undergo, endothermic decomposition or dissociation at temperatures between their storage temperature and the turbine and/or combustor entrance temperature are particularly suited for use in the engine. The most suitable fuels will dissociate at these temperatures to hydrogen and other relatively low molecular weight compounds without the formation of free carbon particles. Preferred examples of such fuels are ammonia, methyl alcohol and cyclohexane. t

The preferred high heat capacity fuels, ammonia, methyl alcohol, ethylene glycol and cyclohexane, dissociate into hydrogen and nitrogen, hydrogen-and carbon monoxide, hydrogen and carbon monoxide, and hydrogen and benzene, respectively. The dissociation is endothermic and the resulting gaseous products are of low molecular weight and are exceptionally clean; that is, they and their combustion products have little or no tendency to foul the engine as they contain no free carbon.

In general, the invention comprises a reaction propulsion system including means providing a comtake, means directing air from the ram air intake to the forward end of the combustion chamber, a fuel storage chamber, indirect heat exchange means in heat exchange contact with the ram air insaid air directing means, means directing fuel from said storage chamber through said heat exchange means, an air compressor providing a portion of the air directing means between the heat exchange means and the forward end of the combustion chamber, a direct expansion turbine for driving said compressor, a regenerative heat exchanger positioned in an auxiliary combustion chamber, means for selectively directing at least a portion of the fuel passing through said heat exchange means to said regenerative heat exchanger, means directing at least a portion of the fuel from said heat exchange means and the fuel passing through the regenerative heat exchanger through the direct expansion turbine, and means for selectively combusting thefuel exhausting from the direct expansion turbine in the combustion chamber and in the auxiliary combustion chamber housing the regenerative heat exchanger.

The invention will be more particularly described with reference to the illustrative embodiments thereof shown in the drawings wherein:

FIG. 1 is a schematic sectional view of a reaction propulsion engine embodying the principles of the present invention;

FIG. 2 is a section substantially on line 2-2 of FIG.

FIG. 3 is a section substantially on line 3-3 of FIG. 1; and

FIG. 4 is a schematic sectional view of a reaction propulsion engine similar to that illustrated in FIG. 1 showing a modif ed form of the present invention.

Referring to the drawings and, in particular, FIGS. 1 through 3, 10 generally designates a reaction propulsion engine embodying the principles of the present invention. The engine includes a shell 12 having a forward end 14 and a rearward end 16. The forward end 14 of the shell or casing 12 connects to the inlet duct for ram air, the path of the ram air being indicated by the directional arrows A.

In the forward end 14 of the ram air passage is an indirect heat' exchange means 18 having its outer surface in contact with the ram air entering the engine. The internal surfaces of the indirect heat exchange means provides a path for the flow of fuel. Fuel from the fuel tank 20 is directed via conduit 22 through turbine driven pump 24 and conduit 26 to the indirect heat exchange means 18. The rate of flow of fuel to the ram air heat exchanger 18 may be controlled within wide limits by varying the output from the fuel pump 24 or by providing a control valve 21 in the fuel line. The pump output volume or the control valve 21 may be manually controlled and/or as indicated in the drawing the control thereof may be provided by the compressor outlet temperature sensing means 23 which would insure that sufficient fuel is passed to the heat exchanger 18 to maintain limits on the turbine inlet temperature and the compressor discharge temperature. From the indirect heat exchange means 18 a conduit 28 directs the fuel to a fuel flow control valve 30. From the fuel control valve 30, the fuel or a portion thereof may be directed to conduit 32 connecting the control valve with a direct expansion turbine nozzle ring 34 which directs the fuel to the turbine blades of the direct expansion turbine 36.

A portion of the fuel directed to the fuel flow control valve 30 may also be selectively directed via conduit 38 to a regenerative heat exchange means 40 positioned in auxiliary combustion chamber 41 about the external surface of the combustion chamber 42 of the reaction propulsion engine. The return line 44 from the regenerative heat exchange means 40 flows to the flow control valve 30 where it is mixed with the portion of the fuel flowing from the valve means through conduit 32 to the annular nozzle ring 34 of the turbine 36. The return line 44 or the control valve 30 is provided with flow restricting means to produce a pressure drop between conduit 38 and return line 44 from the regenerative heat exchanger 40. Also a portion of the fuel flowing from the indirect heat exchange means 18 to the flow control valve means 30 may be directed via conduit 46 to fuel outlet conduits means 72a, b, c, and d positioned in the rearward portion of the combustion chamber 42. Instead of connecting conduit 46 to the fuel outlet conduit means 72a, 12, c, and d, conduit 46 may be connected to a thrust nozzle, not shown in the drawings. The thrust nozzle may be positioned in the combustion chamber or externally thereof. The use of an externally positioned thrust nozzle has particular utility where the amount of fuel needed to cool the ram air is greater than the amount which could be burned stoichiometrically with the available air supply.

The fuelissuing from the turbine 36 is selectively directed, by valve plate means 62 into the forward end of the auxiliary combustion chamber 41, or into the combustion chamber 42, or into both the forward end of the auxiliary combustion chamber 41 and the combustion chamber 42 in selected proportions. The compressed air issuing from the air compressor 54 flows through annular chamber 61 into the combustion chamber 42. Further, a portion of this compressed air is directed into the auxiliary combustion chamber 41 via inlet passage 63. The compressed air entering the auxiliary combustion chamber 41 or combustion products of fuel and the compressed air issue from the auxiliary combustion chamber 41 into the combustion chamber 42 through outlets 65 adjacent the rear end of the auxiliary combustion chamber 41 as to be more fully described hereinafter.

Referring particularly to FIGS. 2 and 3 of the drawing, the valve plate means 62 is provided with a plurality of arcuate ports 64a, b, c, and d and 66a, b, c, and d.

Port 64a cooperates with a fuel outlet conduit 68a provided with a plurality of fuel outlet nozzles 70. Likewise, port 64b cooperates with fuel outlet conduit 68b; port 64c cooperates with fuel outlet conduit 68c; and fuel outlet port 64d cooperates with fuel outlet conduit 68d.

Outlet port 66a cooperates with fuel outlet conduit 72a provided with a plurality of outlet ports or nozzles 74. Similarly, valve port 66b cooperates with fuel outlet conduit 72b; fuel outlet valve port 66c cooperates with fuel outlet conduit 72c; and valve port 66d cooperates with fuel outlet conduit 72d.

The valve plate 62 is rotatably mounted within the engine housing and is selectively movable by control means generally designated 76 whereby all of the fuel exhausting from the turbine 36 may be directed through valve ports 64a, b, c, and d into fuel outlet conduits 68a, b, c, and d; all of the fuel exhausting from the turbine 36 may be directed through valve ports 66a, b, c, and d to fuel outlet conduits 72a, b, c, and d; or a portion of the fuel may be directed through valve ports 64a, b, c, and d to fuel outlet conduits 68a, b, c, and d and the other portion may be directed through valve ports 66a, b, c, and d to fuel outlet conduits 72a, b, c, and d as to be more fully described hereinafter.

It will be particularly noted from FIG. 1 of the drawings that fuel outlet conduits 68a, b, c, and d discharge into the auxiliary combustion chamber 41 while fuel outlet conduits 72a, b, c, and d are spaced downstream thereof within the combustion chamber 42 and that the regenerative indirect heat exchanger 40 is positioned in the auxiliary combustion chamber 41.

As hereinbefore discussed, for high cooling efficiency and to provide a high absorption of energy per pound of fuel, it is desirable that the fuel selected have a high heat capacity within the expected operating temperature range of the engine. At the same time for efficient conversion of energy into thrust in the expansion process, the fuel should be one which provides low average molecular weight components in the exhaust gases. Thus, cryogenic hydrogen or fuels which undergo endothermic decomposition at temperatures between their storage temperature and the turbine and/or combustor entrance temperatures are particularly suited for use in the system of the invention.

When such fuels are used at relatively low flight speeds below about Mach L5 in a system where the incoming air and a regenerative heat exchanger are used to heat the fuel prior to combustion, there will be very little heating of the fuel by the incoming air. Most of the heat required to decompose and/or evaporate the fuel and heat the fuel to the turbine inlet temperature required to operate the air compressor will come from the regenerative heat exchanger. As the flight speed and the temperature of the inlet air increase, there will be more cooling of the air ahead of the compressor and hence more heating of the fuel prior to its passage to the turbine and/or auxiliary combustion chamber. Thus, less heat will be required from the auxiliary combustion chamber via the regenerative heat exchanger and less fuel will be programmed through the regenerative heat exchanger located in the auxiliary combustion chamber prior to its expansion across the turbine. At still higher flight speeds, all of the required heat will come from the cooling of the inlet air and no heat will be taken from the auxiliary combustion chamber via the regenerative heat exchanger.

The air to fuel indirect heat exchanger located ahead of the air compressor serves three main'purposes. The first is toincrease the available turbine work of the fuel by heat addition without combustion while lowering the work required to compress the air thereby making the cycle more efficient. The secondis to.cool the incoming air to acceptable temperature levels, preferably passing therethrough and burnout of the regenerative heat exchanger 40. Further, as less fuel is needed to drive the turbine 36, as the temperature of the fuel below l',000 F., to avoid excessive exit temperatures and dissociation of the products in the combustion zone.,The third is to increase the density of the air through cooling to give a higher mass flow per unit of compressor frontal area and a resultant higher level of thrust.

At higher flight Mach numbers, it becomes increas-' ingly important to cool the incoming air, down to acceptable .levels for the compressor. It then becomes necessary to runtheheat exchanger richer .than at lower Mach numbers, i.e., using an amount of fuel in excess of that which can be expanded across the turbine without overspeeding the rotating assembly. The

excess fuel is passed directly to thecombustor for combustion along with the; fuel that has been expanded across the turbine. A portion of the fuel used in cooling the inlet air may be burned stoichiometrically with the air in the combustor while; the remainder maybe expanded directly to the atmosphere through a separate thrust nozzle as discussed hereinbefore. The proportion separately expanded is determined by the maximumexhaust gas temperatures and the maximum degree of dissociation to be maintained. The use of endothermically dissociating fuels according to the process of our invention minimizes the expenditure of uncombusted fuel.

EXAMPLE An engine of the type illustrated in FIGS. 1 through 3 can be operated efficiently through the velocity range from static launch to Mach l and altitudes up to 150,000 feet using the endothermically dissociating fuels of the invention or cryogenic hydrogen. Below Mach 4 insufficient heat is transferred from the incoming ram air to the fuel in the heat exchanger 18 toeffectively operate the turbine which, in turn, drives the air compressor 54 and the fuel pump 20. Under these conditions, the flow control valve means directs a portion of the fuel passing to the valve from the heat exchanger 18 to'conduit 38 for passage through the regenerative heat exchanger 40. Under these conditions, the control means 76 for the flow control valve plate 62 adjusts the valve ports 64a, b, c, and d and 66a, b, c, and d such that all of the fuel exhausting from the turbine 36 flows through valve ports 64a, b, c, and d and into fuel outlet conduits 68a, b, c, and d whereby the fuel is burned in the auxiliary combustion chamber 41 containing the regenerative heat exchanger 40. As the speed of the vehicle increases and the temperature of the ram air passing the heat exchanger 18 increases, less and less of the fuel is directed, by flow control valve means 30, to the regenerative heat exchanger 40 and proportionately less of the fuel exhausting from the turbine 36 is directed to the fuel outlet conduits 68a, b, c, and d and a greater portion is discharged through valve ports 66a, b, c, and d into the fuel outlet conduits 72a, b, c, and d. The fuel issuing from the fuel outlet conduits 720, b, c, and d is burned within the combustion chamber remote from the regenerative heat exchanger which prevents overheating of. the fuel passing to the turbine increases, the flow control valve means 30 directs excess fuel via conduit 46 directly into the combustion chamber via the open ends of outlet conduits 72a, b, c, and d. The controllers for fuel flow control valve means 30 and controller 76 for valve plate means 62 may be fuel, turbine or compressor temperatures responsive, altitude responsive, vehicle speed responsive, turbine speed responsive or a combination of two or more of these factors, or the valves may be connected to mechanical or electronic programming means which may be of the ground control orof the pre-set type as is known in the art. About Mach 4 none of the fuel passing from the heat exchanger 18 is passed to the regenerative heat exchanger 40 and the inlet air heating of the fuel providesall of the energy requirements of the turbine.

' In the form of the invention illustrated in FIGS. 1 through 3 a portion of the fuel is burned directly in the auxiliary combustion chamber 41 containing the regenerative heat exchanger and a portion of the compressed air is bypassed therethrough. Further, the combustion products or the compressed air not combusted in the auxiliary combustion chamber 41 discharges directly into the primary combustion chamber 42 and adds to the thrust of the vehicle. Similar results are obtainable where the regenerative heat exchanger is in an auxiliary combustion chamber and a portion of the combustion products from the primary combustion chamber are bypassed selectively through the auxiliary combustion chamber. This form of the invention is illustrated in FIG. 4, and referring to FIG. 4, the engine includes a shell or casing 112 having a forward end 114 and a rearward end 116. The forward end 114 of the shell connects to the inlet duct for ram air; the path of the ram air being indicated by the directional arrows A.

In the forward end 114 of the ram air passage is an indirect heat exchange means 1 18 having its outer surface in contact with the ram air entering the engine. Fuel from the tank is directed via conduit 122 through turbine driven pump 124 and conduit 126 to the interior of the indirect heat exchange means 118. The rate of flow of fuel may be controlled by valve means 121 and temperature sensing means 123 as 'directed in reference to FIG. 1 of the drawing. From the indirect heat exchange means 118 a conduit 128 directs the fuel to a fuel, flow control valve 130. From the fuel control valve 130, the fuel or a portion thereof may be directed to conduit 132 connecting the control valve with a direct expansion turbine nozzle ring 134 which directs the fuel to the turbine blades of the direct expansion turbine 136.

A portion of the fuel directed to the fuel flow control valve may also be selectively directed via conduit 138 to a regenerative heat exchange means 140 positioned in auxiliary combustion chamber 141 about the external surface of the combustion chamber 142 of the reaction propulsion engine. The return line 134 from the regenerative heat exchange means 140 flows through a flow restrictor, then to the flow control valve 130 where it is mixed with a portion of the fuel flowing from the valve means through conduit 132 to the annular nozzle ring 134 of the turbine 136. Also a portion of the fuel flowing from the indirect heat exchange means 118 to the flow control valve means 130 may be directed via conduit 146 to fuel outlet conduits 148 provided with fuel outlet openings 150. As described with reference to FIG. 1 of the drawings, conduit 146 may be connected to a thrust nozzle positioned within the combustion chamber 142 or positioned externally of the combustion chamber 142. The fuel outlet conduits 148 are positioned in the rearward portion of the combustion chamber 142.

The direct expansion turbine 136 is connected through shaft 150 to the rotor 152 of an air compressor 154. 'Shaft 156 extending from the forward end of the rotor 152 of the air compressor drives the fuel pump 124.

The fuel issuing from the turbine 136 is directed into the combustion chamber 142 through the fuel outlet conduits 148 and outlet nozzles or openings 150 therein. Thus, both fuel passing through the turbine 136 and fuel bypassing the turbine may be conveniently directed into the combustion chamber through the same outlet nozzles.

Downstream of the outlet nozzles 150 is a plurality of openings 152 in the peripheral wall of the combustion chamber. Openings 152 permit the flow of combustion products, uncombusted fuel, and/or compressed air into the auxiliary combustion chamber 141 which houses the regenerative heat exchanger 140. The combustion products, fuel and/or compressed air entering the auxiliary combustion chamber 141 through inlets 152 flow past the regenerative heat exchanger 140, thence back into the primary combustion chamber 142 via passages 154 at the rearward end of the primary combustion chamber.

In order to control the flow of the combustion products through the auxiliary combustion chamber 141 containing the regenerative heat exchanger 140, valve means illustrated as an annular band 156 provided with a plurality of openings 158 therein is adjustably mounted about the external surface of the combustion chamber 142 in line with the openings 152 which provide communication between the interior of the combustion chamber 142 and the auxiliary combustion chamber 141. The openings 158 in the band 156 cooperate with the openings 152 in the peripheral surface of the combustion chamber and regulate the flow of the combustion products into the auxiliary combustion chamber 141.

Movement of the band type valve means 156 may be controlled by control means such as generally designated 76 in the form of the invention illustrated in FIGS. 1 through 3, whereby the passages 152 would be open to the auxiliary combustion chamber 141 during low flight speeds and the passages would be progressively closed as the vehicle approaches high speed operation and fully closed at about Mach 4 at which speed all of the energy requirements of the direct expansion turbine 136 would be provided by the indirect heat exchange means 118 in the ram air passage.

Where desired, the cold fuel exhausting from the turbine may be utilized to cool engine parts such as the combustor case, exhaust nozzle, etc. prior to combustion of the fuel with the compressed air in the combustor.

We claim:

1. A reaction propulsion system including means providing a combustion chamber having at its rearward end an impulse expansion outlet nozzle, means providing a ram air intake, means directing air from the ram air intake to the forward end of the combustion chamber, a fuel storage chamber, indirect heat exchange means in heat exchange contact with the ram air in said air directing means, means directing fuel from said storage chamber through said heat exchange means, an air compressor providing a portion of the air directing means between the heat exchange means and the combustion chamber, a direct expansion turbine for driving said compressor, a regenerative heat exchanger positioned in an auxiliary combustion chamber, means for selectively directing at least a portion of the fuel passing'through said heat exchange means to said regenerative heat exchanger, means directing at least a portion of the fuel from said heat exchange means and the fuel passing through the regenerative heat exchanger through the direct expansion turbine, and means for selectively combusting the fuel exhausting from the direct expansion turbine in the combustion chamber and in the auxiliary combustion chamber housing the regenerative heat exchanger.

2. A reaction propulsion engine including means providing a combustion chamber having at its rearward end an impulse expansion outlet nozzle, means providing a ram air intake, means directing air from the ram air intake to the forward end of the combustion chamber, a fuel storage chamber, indirect heat exchange means in heat exchange contact with the ram air in said ram air directing means, means directing fuel from said storage chamber through said heat exchange means, an air compressor providing a portion of the air directing means between the heat exchange means and the forward end of the combustion chamber, a direct expansion turbine for driving said compressor, a regenerative heat exchanger positioned in an auxiliary combustion chamber, means for selectively directing at least a portion of the fuel passing through said heat exchange means to said regenerative heat exchanger, means for directing a portion of the fuel passing through the heat exchange means to the combustion chamber, means for directing the other portion of the fuel from said heat exchange means and the fuel passing through the regenerative heat exchanger through the direct expansion turbine, and means for selectively combusting the fuel from the direct expansion turbine in the combustion chamber and in the auxiliary combustion chamber housing the regenerative heat exchanger.

3. The invention defined in claim 1 wherein a portion of the compressed air from the air compressor is selectively directed to the auxiliary combustion chamber containing the regenerative heat exchanger together with a portion of the fuel exhausting from the direct expansion turbine, and means for directing the products of combustion exhausting from the auxiliary combustion chamber into the rearward end of the combustion chamber.

4. The invention defined in claim 1 wherein the means for selectively combusting the fuel exhausting from the direct expansion turbine in the auxiliary combustion chamber having the regenerative heat exchanger comprises a valved passage connecting the combustion chamber and the auxiliary combustion chamber and further passage means directing combustion products issuing from the auxiliary combustion chamber into the combustion chamber. 

1. A reaction propulsion system including means providing a combustion chamber having at its rearward end an impulse expansion outlet nozzle, means providing a ram air intake, means directing air from the ram air intake to the forward end of the combustion chamber, a fuel storage chamber, indirect heat exchange means in heat exchange contact with the ram air in said air directing means, means directing fuel from said storage chamber through said heat exchange means, an air compressor providing a portion of the air directing means between the heat exchange means and the combustion chamber, a direct expansion turbine for driving said compressor, a regenerative heat exchanger positioned in an auxiliary combustion chamber, means for selectively directing at least a portion of the fuel passing through said heat exchange means to said regenerative heat exchanger, means directing at least a portion of the fuel from said heat exchange means and the fuel passing through the regenerative heat exchanger through the direct expansion turbine, and means for selectively combusting the fuel exhausting from the direct expansion turbine in the combustion chamber and in the auxiliary combustion chamber housing the regenerative heat exchanger.
 2. A reaction propulsion engine including means providing a combustion chamber having at its rearward end an impulse expansion outlet nozzle, means providing a ram air intake, means directing air from the ram air intake to the forward end of the combustion chamber, a fuel storage chamber, indirect heat exchange means in heat exchange contact with the ram air in said ram air directing means, means directing fuel from said storage chamber through said heat exchange means, an air compressor providing a portion of the air directing means between the heat exchange means and the forward end of the combustion chamber, a direct expansion turbine for driving said compressor, a regenerative heat exchanger positioned in an auxiliary combustion chamber, means for selectively directing at least a portion of the fuel passing through said heat exchange means to said regenerative heat exchanger, means for directing a portion of the fuel passing through the heat exchange means to the combustion chamber, means for directing the other portion of the fuel from said heat exchange means and the fuel passing through the regenerative heat exchanger through the direct expansion turbine, and means for selectiveLy combusting the fuel from the direct expansion turbine in the combustion chamber and in the auxiliary combustion chamber housing the regenerative heat exchanger.
 3. The invention defined in claim 1 wherein a portion of the compressed air from the air compressor is selectively directed to the auxiliary combustion chamber containing the regenerative heat exchanger together with a portion of the fuel exhausting from the direct expansion turbine, and means for directing the products of combustion exhausting from the auxiliary combustion chamber into the rearward end of the combustion chamber.
 4. The invention defined in claim 1 wherein the means for selectively combusting the fuel exhausting from the direct expansion turbine in the auxiliary combustion chamber having the regenerative heat exchanger comprises a valved passage connecting the combustion chamber and the auxiliary combustion chamber and further passage means directing combustion products issuing from the auxiliary combustion chamber into the combustion chamber. 